Experience with 54 patients affected by pulmonary carcinoma treated or not with surgery and undergoing thymostimulin administration during long-term follow-up (70 mg i.m. every other day for 3 months), is reported. Drug intolerance was observed in 5.5% of cases. In patients who were able to complete the therapeutic cycle (50 cases) objective improvement of Performance Status was obtained in 46% of cases and subjective improvement in nearly 90%. The course of neoplastic disease showed definite progression (presence of local recurrence or distant metastasis) in 20% of cases, remission in 6%. No case of onset of pulmonary or extrapulmonary infections was observed. After treatment, a significant increase (between 24% and 108%) in blood parameters (circulation lymphocytes, CD3, CD4, CD8, CD16, IgG, IgA, IgM) was observed in 28-56% of cases. As for CD4 increase, this was accompanied by concomitantly positive Merieaux test in 44.5% of cases. Quiescence or complete remission has appeared to occur together with high CD16 values, progression with high CD8 and low CD16 values.